1. Field of the Invention
The present invention generally relates to a surface finish structure of multi-layer substrate and manufacturing method thereof, and more particularly, to a surface finish structure of a flexible multi-layer substrate and manufacturing method thereof.
2. Description of Prior Art
According to prior arts, a surface finish structure of multi-layer substrate can be categorized into two methods, i.e. Pad Definition and Solder Mask Definition.
Please refer to FIG. 1, which depicts a diagram of surface finish structure in Pad Definition according to prior arts. After a bond pad layer 101 is formed on a dielectric layer, a solder mask 104 is coated. Next, after a hole is made at the position of the bond pad layer 101, a necessary step called “descum” is performed for cleaning the remaining scum. After that, the cover metal layer of nickel 102 and the cover metal layer of gold 103 are formed on the bond pad layer 101.
Please refer to FIG. 2, which depicts a diagram of surface finish structure in Solder Mask Definition according to prior arts. After a bond pad layer 101 is formed on a dielectric layer, a solder mask 104 is coated. Next, after a hole is made at the position of the bond pad layer 101, a necessary step called “descum” is performed for cleaning the remaining scum. After that, the cover metal layer of nickel 102 and the cover metal layer of gold 103 are formed on the bond pad layer 101. The difference between the Solder Mask Definition method and the Pad Definition method shown in FIG. 1 is that the area of the hole in the Pad Definition method is large enough to expose the whole bond pad layer 101 but solder mask 104 covers a portion of the bond pad layer 101 in the Solder Mask Definition method.
In both the Solder Mask Definition method and the Pad Definition method, the cover metal layers must be formed after first coating the solder mask 104 and making the hold. When a device is connected to the bond pad layer 101, which is generally of copper, solder may contact the copper and a wetting phenomenon may occur. The purpose of forming the cover metal layers is to prevent the contact of the bond pad layer 101 with the solder. However, delamination may occur at places of the arrow points shown in FIG. 1 or FIG. 2 due to humidity in the environment, stress between the cover metal layer and the dielectric layer or stress between the cover metal layer and the bond pad layer because they are made of different materials. Accordingly, the solder may contact with the bond pad layer 101 and then fragile IMC (Intermatallic Compound Layer) may appear to cause the contact point to be weakened and thus low down the reliability of the production made by the multi-layer substrate.
Moreover, regardless of the method is either the Solder Mask Definition method or the Pad Definition method, the bond pad layer 101 is formed on the dielectric surface, therefore, bond pad layer 101 may have chance to peel or delaminate from the dielectric layer. Accordingly, package reliability may become lowered.
Therefore, preventing contact of the bond pad layer with solder when the package is processed and enhancing the bond pad layer adhered to the dielectric layer below can raise reliability of package and the yield of the packaged production.